Shale gas production operations throughout the United States and all over the world increasingly rely on hydraulic fracturing as a completion process to stimulate natural gas production from shale formations. Hydraulic fracturing involves high-pressure injection of large quantities of water, along with sand and other small amounts of chemical additives, into a well. The high-pressure water creates small fractures, or cracks, in the surrounding rock formation, and sand or other propant used wedges into the cracks and prevents them from closing up once the water pressure is removed. These cracks allow any gas entrapped in the shale formation to escape and be recovered at the wellhead, and ultimately sold to consumers.
Upon completion of a hydraulic cracking process in a well, as a result of discontinuing the high-pressure water injection a large portion of the injected water (from several thousand to millions of gallons) is recovered at the wellhead as flowback water. Such flowback water typically contains liquid hydrocarbons, gases dissolved in the water due to the high-pressures in the well, other organic or non-organic components, and solids which may include the propant injected in the well, as well as sand/sediment, drill cuttings, and soil, washed up by the flowback water.
The gases dissolved in the flowback water as a result of the high pressures created during the hydraulic injection are highly flammable and pose serious environmental hazards. Hazardous air pollutants (HAP or HAPs) typically dissolved in or mixed with the flowback water comprise greenhouse gases, volatile organic compounds (VOC or VOCs), such as nitrogen oxides (NOx), methane (CH4), nitrous oxide (N2O), carbon dioxide (CO2), and liquid hydrocarbons, such as formaldehyde, benzene, and chlorofluorocarbons (CFCs). VOCs are organic chemicals that have a high vapor pressure at ambient conditions, and are especially problematic due to their tendency to readily dissolve in, or mix with, flowback water at high pressures, and to also readily evaporate from flowback water at atmospheric pressures and ambient temperatures. Further, most VOCs are generally not immediately toxic, but instead have compounding long-term health effects on oilfield personnel and other persons and animals exposed to even low concentrations of VOCs for prolonged periods of times.
Due to the health and environmental hazards of HAPs and VOCs, strict environmental laws and regulations have been enacted on the state (e.g., California Air Resources Board), federal (e.g., EPA, OSHA), and international levels, to minimize the emissions of HAPs, and VOCs in particular, into the environment during oilfield and other industrial and agricultural operations.
Conventional flowback water handling practice has been to flow wellstream fluids (e.g., flowback water) through a gas production unit or into a direct-fired production separator for a three-phase separation of the sales gas, the condensate/oil, and the flowback water at sales gas pressure. Conventional gas production units operate at pressures between 100-1400 psig and typically dump the exiting flowback water directly into atmospheric storage tanks after the sales gas has been separated.
Several problems are inherent in this approach, including inadequate retention times in the production separator and the flashing off of gas at the condensate and flowback atmospheric tanks resulting from the large pressure drop (e.g. from about 1400 psig to atmospheric pressure). Flash gases vented at flowback water and condensate storage tanks pose severe dangers of fires/explosions and environmental hazards of substantial amounts of HAPs emissions including VOCs.
To this end, a need exists for a method and separator assembly for treating large volumes of flowback fluid inherent in hydraulic fracturing operations without allowing substantial amounts of VOCs and HAPs to be emitted to the atmosphere. It is to such a method and separator assembly that the inventive concepts disclosed herein are directed.